1. Field of the Invention
The present invention relates to a color-image forming apparatus, an image forming method, and a computer program product.
2. Description of the Related Art
In accordance with market demand, electrophotographic devices that can output color images, such as color copiers and color printers, are used more and more. Especially, with the demand for color-image outputting speeds as high as monochrome-image outputting speeds, tandem-type color-image forming apparatuses that include color-based photosensitive elements and color-based developing devices have recently become mainstream. The tandem-type color-image forming apparatuses form toner images with different single colors on individual photosensitive elements and then sequentially transfer the single-color toner images onto a transfer sheet, thereby recording a color image (see, for example, Japanese Patent Application Laid-open No. 2006-126643).
In a typical tandem-type color-image forming apparatus, regardless of whether it is a direct-transfer type or an intermediate-transfer type, the single-color images are transferred from the individual photosensitive elements, at different positions on an intermediate transfer belt, to the intermediate transfer belt or the transfer sheet; therefore, even a fine change in the moving speed of the intermediate transfer belt alters the timing at which the intermediate transfer belt reaches the transfer position of the next single-color image, which in turn causes the transfer positions of the single-color images to shift from the correct positions, which results in an output image with misalignment (color shift) in the sub-scanning direction.
A typical tandem-type color-image forming apparatus includes writing units separated from each other on the basis of color. If the constituents are displaced from the correct positions due to an environmental change, such as a temperature change, which in turn changes the magnifying power and the writing position, an output image with misalignment (color shift) in the main-scanning direction is formed.
To prevent misalignment, a typical tandem-type color-image forming apparatus forms a specific pattern image for positional alignment on the intermediate transfer belt between the image processing area for a first page and the image processing area for a second page. A typical tandem-type color-image forming apparatus detects misalignment (color shift) in both the main-scanning direction and the sub-scanning direction using the pattern image and performs positional alignment to correct the detected misalignment (color shift).
However, because the above-described positional alignment needs a given processing time, a period of downtime during which the printing process cannot be performed occurs, which decreases the printing performance. Moreover, if, because of the timer setting or the like, the positional alignment interrupts the monochrome printing that does not require a positional alignment, although the positional alignment is not needed, the monochrome printing is interrupted and thus the printing performance decreases.
Japanese Patent Application Laid-open No. 2006-126643 discloses a technology that prevents the decrease in the printing performance caused by the positional alignment. If an engine control unit receives a print job from a controller unit before the start of the positional alignment, the engine control unit delays the positional alignment. If the engine control unit receives a print job during the positional alignment, the engine control unit suspends the positional alignment and starts the print job.
However, according to the technology disclosed in Japanese Patent Application Laid-open No. 2006-126643, because the intermediate transfer member is not able to perform printing during the positional alignment, the problem of the decrease in the printing performance cannot be solved.